19 It may seem waste of time to bring forward further arguments, for truths concerning God gain no strength by repetition; a single statement suffices to establish them. Yet it is well for us to know all that has been revealed upon the subject, for though we are not responsible for the words of Scripture, yet we shall have to render an account for the sense we have assigned to them. One of the many commandments which God gave to Noah is, Whoso sheds man's blood, for his blood shall his life be shed, for after the image of God made I man. Here again is the distinction between likeness, creature, and Creator.
God bears witness that He made man after the image of God. When He was about to make man, because He was speaking of Himself, yet not to Himself, God said, After our image; and again, after man was made, God made man after the image of God. It would have been no inaccuracy of language, had He said, addressing Himself, I have made man after My image, for He had shown that the Persons are one in nature by, Let us make man after Our image. But for the more perfect removal of all doubt as to whether God be, or be not, a solitary Being, when He made man He made him, we are told, After the image of God.
Source: On the Trinity (New Advent)